Ever since carbon nanotubes were first discovered by Iijima, et al. in 1991 (S. Iijima, Nature, 354, 56 (1991)), their physical properties have been evaluated and their functions have been elucidated, and extensive research and development have been conducted on their application. However, since carbon nanotubes are produced in an entangled state, they have the shortcoming of being extremely bothersome to handle. In the case of mixing into resins and solutions, there is also the problem of the carbon nanotubes becoming increasingly aggregated, thereby preventing them from demonstrating their inherent characteristics.
Consequently, attempts have been made to uniformly disperse or solubilize carbon nanotubes in solvents or resins by subjecting them to physical treatment or chemical modification.
For example, a method has been proposed in which single-walled carbon nanotubes are cut into short pieces and dispersed by subjecting to ultrasonic treatment in strong acid (R. E. Smalley, et al., Science, 280, 1253 (1998)). However, since treatment is carried out in strong acid, the procedure is complex and not suitable for industrial applications, while the dispersion effects cannot be said to be adequate.
Therefore, by noticing that both ends of single-walled carbon nanotubes cut in the manner proposed above are open, and that they are terminated with oxygen-containing functional groups such as carboxylic acid groups, it was proposed that carbon nanotubes be made soluble in solvent by introducing long-chain alkyl groups by reacting with an amine compound after having converted the carboxylic acid groups into acid chloride (J. Chen, et al., Science, 282, 95 (1998)). However, in this method, since long-chain alkyl groups are introduced into single-walled carbon nanotubes by covalent bonding, there was still the problem of damage to the graphene sheet structure of the carbon nanotubes and effects on the characteristics of the carbon nanotubes itself.
Another attempt to produce water soluble single-walled carbon nanotubes was reported that comprising introducing substituents containing ammonium ions in pyrene molecules by utilizing the fact that pyrene molecules are adsorbed onto the surfaces of carbon nanotubes by strong interaction, and subjecting these to ultrasonic treatment in water together with single-walled carbon nanotubes to non-covalently adsorb them to the single-walled carbon nanotubes (Nakajima, et al., Chem. Lett., 638 (2002)). According to this method, although damage to the graphene sheet structure is inhibited due to the non-covalent bonding chemical modification, since non-conducting pyrene compounds are present, there is the problem of a decrease in the conductivity of the resulting carbon nanotubes.